As a method for producing hydrocarbons having a broad carbon number distribution, which can be used as feedstocks for liquid fuel products such as kerosene and gas oil, methods that employs the Fischer-Tropsch synthesis reaction (hereinafter also abbreviated as the “FT synthesis reaction”), which uses carbon monoxide gas (CO) and hydrogen gas (H2) as a feedstock, are already well known. Moreover, techniques which use natural gas or the like as a feedstock to produce a synthesis gas (a mixed gas containing CO and H2 as the main components) via a reforming process, subsequently use the FT synthesis reaction to synthesize hydrocarbons from the synthesis gas, and then subject these hydrocarbons to hydrotreating and fractional distillation to produce liquid fuels are known as GTL (Gas to Liquids) processes.
Patent Document 1 discloses one example of a method for producing hydrocarbons by the FT synthesis reaction, the method using a slurry bubble column reactor in which a synthesis gas is blown through a slurry prepared by suspending solid catalyst particles within a liquid hydrocarbon (hereinafter simply referred to as “the slurry”) to effect the FT synthesis reaction.
In this method, the hydrocarbons are produced using a reaction system that includes a reactor (slurry bubble column reactor), which houses the slurry and in which the FT synthesis reaction is performed, a supply line (gas supply section) that blows the synthesis gas into the bottom of the reactor, a filter that separates the catalyst particles from the slurry from inside the reactor, a discharge line for discharging the hydrocarbons (heavy liquid hydrocarbons) that have been synthesized inside the reactor and passed through the filter, and a mechanism that returns, to the reactor, a portion of the liquid hydrocarbons discharged through the discharge line, thereby washing the filter.
Further, the above mechanism for washing the filter, namely the mechanism for returning to the slurry the accumulated catalyst particles trapped by the filter, typically employs a backwash treatment in which a portion of the liquid hydrocarbons (heavy liquid hydrocarbons) that have been filtered through the filter and discharged through the discharge line is caused to flow back through the filter in the opposite direction to the flow direction during filtering of the slurry.
On the other hand, a gas phase portion is provided above the slurry inside the reactor, and the synthesis gas that has not reacted during passage through the slurry (namely, unreacted synthesis gas) and light hydrocarbons produced by the FT synthesis reaction that are gases under the conditions inside the reactor are transported from the slurry bed into this gas phase portion, subsequently pass through the gas phase portion, and are then discharged through a line connected to the top of the reactor. The discharged unreacted synthesis gas and light hydrocarbons are cooled, some of the light hydrocarbons condense to form liquid hydrocarbons (light liquid hydrocarbons), and these light liquid hydrocarbons are separated from the gas fraction (the unreacted synthesis gas and a hydrocarbon gas composed mainly of hydrocarbons of C4 or less) by gas-liquid separation. The gas fraction is then recycled through the reactor to enable the unreacted synthesis gas to be reused, while the light liquid hydrocarbons are supplied to a liquid hydrocarbon refining process.